The Optical Transport Network (OTN) is defined by a series of recommendations or standards coordinated by the International Telecommunication Union (ITU). ITU-T Recommendation G.873.1 defines the Automatic Protection Switching (APS) protocol and protection switching operation for the linear protection schemes of the Optical Transport Network at the Optical Channel Data Unit (ODUk) level.
In a linear protection architecture of an optical transport network, protection switching schemes may be generally classified as:                trail protection (at a section or path layer);        sub-network connection protection (which in turn comprises inherent monitoring, non-intrusive monitoring, and sub-layer trail monitoring)        
Sub-network connection protection switching in optical transport networks are further defined in ITU-T Recommendation G.798.
In an optical network, such as a Dense Wavelength Division Multiplexing (DWDM) network, end-to-end protection schemes are offered to customers as Optical Sub-Network Connection Protection (OSNCP), as well as in a Wavelength Switched Optical Network scenario. Further protection schemes based on pre-planned paths can also be defined, such as Safe OSNCP or Transponder Sharing Protection schemes.
FIG. 1 shows an optical network comprising a first node 101, for example a ODUk cross connect node, communicating with a second node 103, for example a ODUk cross connect node, via an optical transport network 105. The first node 101 is shown as comprising an input interface A (for example a traffic card), and first and second output interfaces (or traffic cards) B and C. The second node 103 is shown as comprising first and second input interfaces D and E, and an output interface F. Protection switching in such a network is provided by duplicating an ODUk transmission over two independent paths along the optical transport network. Traffic may be transmitted along the path B→D, which is named the “working path” (W), with a duplicate transmission along the path C→E, which is named the “protecting path” (P). The destination node, or second node 103, will select the traffic (i.e. ODUk traffic) from either the working path W or from the protecting path P depending on quality information. For example, traffic may be selected according to Signal Fail (SF) and Signal Degrade (SD) information detected by the second node 103 at interfaces D and E which receive the working path W and protecting path P traffic, respectively.
In the optical communication networks such as those shown in FIG. 1, since an optical signal is attenuated along a transmission path, it is necessary to amplify an optical signal along its routing path using intermediate nodes, such as optical amplifier devices, transponders or regenerators. For example, in FIG. 1 intermediate nodes 107, 109, 111 are shown in the working path W, and intermediate nodes 113, 115 shown in the protection path P. As such, intermediate nodes in the form of optical amplifier devices, transponders or optical-electrical-optical (OEO) regenerators (also known as 3R regenerators) are used to increase the power of a signal in order to reach a distant node, which can be located a long distance from the source node.
Optical amplifier units are also used on an optical multiplexed section which aggregates a number of optical channel signals, and transporting on an Optical Transport Section (OTS) according to the ITU-T G709 standard.
The optical transport network of FIG. 1 shows a particular route for a working path W and a particular route for the protection path P through the optical transport network 5. It will be appreciated that the working path W and protection path P can be configured to take any route through the optical transport network 5, for example using any combination of a plurality of nodes (not all shown in FIG. 1), the route being determined by a network management system or layer (also not shown).
A disadvantage of existing protection schemes is that the performance of the network in terms of the performance of individual devices involved in the paths is not taken into consideration in the switching criteria of the protection mechanisms. In other words, path protection mechanisms in a DWDM network or WSON based network do not take into consideration the real status of the physical resources involved in the paths.